Resistor made from carbonaceous material

ABSTRACT

A crenellated resistor made from a carbonaceous material and having the shape of a hollow cylinder for use in furnaces working at high temperature. The resistor is characterized by the thickness of the cylinder walls which follows a profile determined so as to produce an even temperature on the surface of the resistor when it is working. Thus, the wall has a radial thickness which decreases in a transition section from the peripheral edge to the internal edge. The resistor configuration allows elimination of hot spots which may appear when electricity flows through the resistor.

This application claims the benefit of Provisional App. No. 60/360,177,filed Mar. 1, 2002.

FIELD OF THE INVENTION

The invention relates to electric resistances used as heating elements,especially those used as a heat source in furnaces working at hightemperature and generally called <<resistors>>. The inventionparticularly relates to such resistors made from carbonaceous materials.

DESCRIPTION OF RELATED ART

Electric resistances, or “resistors”, are well known and commonly usedas heating sources in furnaces working at high temperature. Thematerials used to make these resistances are selected according tovarious criteria such as, notably, the expected working temperature andthe physico-chemical environment surrounding the resistances during use.For furnaces working at a temperature of roughly 1000° C. or above andin an oxygen free atmosphere, it is frequent to use resistances madewith carbonaceous materials, such as graphite, carbon or somecarbon/carbon composites (C/C composites). This choice is justified byseveral properties of these materials such as:

-   -   their ability to withstand high temperatures (up to 2800° C. for        graphite) and/or frequent thermal cycles without undergoing any        transformation or suffering from any loss of properties,    -   electric resistivity values suitable for use in an electric        resistance (especially graphite materials whose resistivities        are typically in the 400 et 5000 μΩ.cm range),    -   a reasonable cost of production.

The electric resistances for furnaces can have different shapes and beassembled in different ways. For example, they can be axisymetrical(apart from the electrical connections), generally in the shape of ahollow cylinder (e.g. U.S. Pat. No. 5,660,752) or of two hollowcylinders of different diameters linked by a tapered connecting portion(e.g. U.S. Pat. No. 4,533,822). For brittle materials such as graphiteand, to a lesser extent, the C/C composites, these resistances are madeby assembling U-shaped sections (U.S. Pat. No. 3,786,165) or shell-likeelements (DE 37 43 879, U.S. Pat. No. 4,549,345, U.S. Pat. No.4,755,858, U.S. Pat. No. 5,660,752). U.S. Pat. No. 6,285,011 disclosedrecently a one-piece almost cylindrical resistor made of carbonaceousmaterial.

These resistors often have the form of a hollow cylinder whose wallcomprises straight, alternated and regularly spaced slots, which form acontinuous path with “crenellations” or meanders, also called“zig-zags”, as illustrated in FIG. 1. Electricity passes along thatpath. This kind of resistor will hereafter be referred to as a“crenellated resistor”.

The meander-like shape of crenellated resistors provides them with highmechanical strength, reduces the number of electric connections andforms an almost continuous wall which emits a very even thermal fluxtowards the inner space of the cylinder. The resistor disclosed by U.S.Pat. No. 6,285,011 is set out as a good compromise between economy ofspace, high mechanical strength and high heating power.

However, when in service, most crenellated resistors develop areas withpronounced temperature gradients (or <<hot spots>>) close to theextremities of each meander. These hot spots go against the temperaturehomogeneity in the inner space of the resistor and can, moreover, createsome undesired thermal gradients on the articles undergoing a heattreatment in said inner space. In addition these hot spots arepotentially highly reactive with the atmosphere surrounding theresistor, mostly because their temperature is higher than those found inthe straight sections of the resistor. This higher reactivity could leadto accelerated physico-chemical transformations affecting some localareas of the resistor (as for instance an oxidation or a siliconizationof the carbonaceous material). These transformations could affect themechanical properties of the material in the most exposed areas(mechanical stresses are concentrated in the curved area). Thesephenomena accelerate the degradation and the wear of the resistors. Forexample, a crenellated resistor used for silicon crystal growing has alifespan of only few months, rarely more than one year.

The inventor has tried to make a resistor for high temperature dutieswhich allows an almost perfectly even temperature distribution in theworking area, which would be free of the drawbacks known on theresistors according to the prior art, which would still have the maincharacteristics of crenellated resistors, which could have an improvedlifespan and which would remain economically competitive.

SUMMARY OF THE INVENTION

The invention provides a crenellated resistor, i.e. a resistance with ahollow cylindrical shape and a meander like design, made from acarbonaceous material, and characterized in that the wall thicknessvaries in the extremities of the meanders—where the electrical lines arecurved—in order to produce an even temperature on the surface of theresistor when in use.

The inventor worked on the hypothesis that the temperature gradientswere due to a high concentration of the electrical flow lines—andtherefore to an increase in the power dissipated per unit of surfacearea—in the part of the meander where the radius of the curve is theshortest. In this way, he tried to increase the electrical resistance inthis area in order to distribute more evenly these electrical flowlines.

The inventor figured out that a modification of the wall thickness ofthe resistor determined so as to modify the electric current density.This modification was mode essentially in the extremities of themeanders where the electrical lines are curved and consisted indecreasing the wall thickness when the radius of the curved electricalline decreased. This idea allows to get an improved temperaturehomogeneity without any major impact on the mechanical characteristicsof the resistor.

The invention is also aimed at the use of a crenellated resistor withits improved features into a furnace working at high temperature. Thisinvention particularly relates to the use of a crenelated resistoraccording to the invention for the production of single crystals—ormonocrystals—of silicon according to the Czochralski process (CZfurnaces). In particular, there is provided a furnace or oven comprisinga crenellated resistor according to the invention.

The invention is particularly useful to pull silicon single crystalsaccording to the Czochralski process. In this application the processreleases a silica rich vapor which reacts with the reactor, especiallyin the hot spots, and creates a premature wear. Besides, thisapplication requires an even heating inside the working area (i.e. inthe inner space of the hollow resistor).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the support of thefollowing figures and detailed description.

FIG. 1 gives a general view of a typical crenellated resistor of priorart.

FIG. 2 schematically illustrates an elementary segment of a crenellatedresistor according to prior art; front face view (A) and cross sectionalview (B).

FIG. 3 gives a simplified view of the electric current line distributioninto an elementary segment of a crenellated resistor according to priorart.

FIG. 4 illustrates an elementary segment of a crenellated resistor madeaccording to the principles of this invention; front face view (A) andcross sectional view (B).

FIG. 5 schematically illustrates an elementary segment of a crenellatedresistor made according to a preferred embodiment of this invention;front face view (A) and cross sectional view (B).

DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated on the FIG. 1, a crenelated resistor (1), with thegeneral shape of a cylinder, generally includes a plurality of straightslots (2), which are alternated and regularly spaced. The slots (2)emerge alternatively at each side (H and B) of the resistor, and thisdefines a path with meanders whose linear resistance is approximatelyeven, at least in the straight portions (3). The turns of each meanderform crenels (or crenellations or meander halves) (6), which will behereafter referred to as “transition sections”.

The crenellated resistor, with a cylindrical shape, determines an insidearea (or inner space) (9) where the parts or products to be treated areloaded. The resistor according to the invention can be an assembly ofseveral pieces, said pieces being for example linked together by partsused also for the electric connection. The crenellated resistoraccording to the invention can also be a one-piece hollow cylinder, asillustrated on the FIG. 1.

Generally, the crenellated resistor includes some electrical and/ormechanical connection means (5, 5′) which typically comprise mereextensions of one or more transition sections.

The thickness of the wall of the crenellated resistor (1) according toprior art (and more precisely the radial thickness relatively to the Caxis of the cylinder) Eo is essentially uniform or, like the resistordisclosed by U.S. Pat. No. 6,285,011, greater at the ends of its section(which is U-shaped or H-shaped).

The length L of a crenellated resistor is typically comprised between afew hundreds of millimeters and 1 meter or even 2 meters. Its diameter Dis typically between 100 and 1000 mm. The wall thickness is typicallybetween 5 and 40 mm.

FIG. 2 exhibits an elementary segment (or meander half) (6) of acrenellated resistor (1), relative to a median line, either seen fromthe front side (2A) or seen along the cross section A–A′ (2B). Thiselementary segment is repeated along the periphery, alternativelyoriented upwards or downwards. The elementary segment typically has twostraight <<legs>> (3) and a transition section (or extremity) (4)linking these two straight legs. The extremities (4) of the elementarysegments can have a more or less perfectly circular shape. When working,the electric current lines (10) are essentially parallel in the straightlegs and gradually change their direction in the transition section (4).FIG. 3 schematically illustrates how the electric current lines (10,10′) are arranged in each elementary segment.

According to the invention, the crenellated resistor (1) made withcarbonaceous material has a wall thickness (or <<radial>> thickness) E,which has in the transition sections a profile adapted to produce aneven surface temperature when an electric current is flowing through it.

More precisely, the crenellated resistor (1) made with a carbonaceousmaterial according to the invention has the shape of a hollow cylinderwhose wall has straight, alternated and regularly spaced slots, whichform a path with meanders, each elementary segment (6) of the resistorcomprising two straight segments (3) and a transition section (4), thesaid transition section (4) comprising a peripheral edge (8) and aninternal edge (7), and is characterized in that, in order to get anapproximately even surface temperature when a current is flowing, theradial thickness E decreases from the peripheral edge (8) to theinternal edge (7).

According to the invention, the wall thickness of the crenellatedresistor is variable in the extremities of its meanders where theelectrical lines are curved, i.e. in the transition sections (4). Saidwall “radial” thickness decreases when the radius of the curvedelectrical line decreases, thus decreases from the peripheral edge (8)to the internal edge (7), in order to distribute more evenly theelectrical flow lines.

Preferably, the wall thickness of the crenellated resistor made from acarbonaceous material decreases in a monotonic manner. Practically, saidmonotonic decrease can be carried out “discontinuously”, i.e. bymachining “steps” on the surface of the resistor (or on the cavitysurface of the mold used for shaping the resistor).

The thickness profile can be defined by a mathematical formula. Forinstance, the thickness profile seen along a line including C′, thecenter of the circular path followed by the transition section (4) (lineT in FIG. 4), could be given by a mathematical function E(x, θ), where θis the angle between the line T and a reference axis (typically the mainaxis Ao of the elementary segment) and x the distance between theconsidered point p, located on the line T and the point C′. The profilefollowed by E is typically a symmetrical function with respect to themain axis Ao of the elementary segment.

The profile of E can be experimentally determined, or through somecalculations (specially finite element calculations) or through amathematical simulation. The surface temperature is linked to theradiated power it emits while working. The inventor has found that thethickness profile could be simply determined through a calculation ofthe power p_(o) dissipated through the Joule effect into any unit ofvolume of the resistor, assuming that every element of the surface willreceive a power equal to the sum of the electric power dissipated ineach the volume units (p_(o)) located beneath it, and will then transferit into its environment. With this principle, the thickness profile ofthe transition section (4) could be determined in a such a way that theelectrical power produced by unit of surface is approximately the samewhatever the surface unit considered.

In a more detailed way, the thickness profile could be determined byassuming that in a steady state situation the power dissipated by eachsurface unit is equal to the electric power Po dissipated below the samesurface unit and using the equation Po=ΔU²/(R×S), where ΔU is thedifference of electric potential between the two extremities of a givensurface unit, R the electrical resistance between these two extremitiesand S the area of the surface unit.

The value of Po can be calculated using the following simplifiedformulas:ΔU=Ua−Ub, R=πrρ/(e×dr) and S=πr×dr,where Ua and Ub are the two equipotential lines <<a>> and <<b>> shown onthe FIG. 4, respectively, ρ is the resistivity of the carbonaceousmaterial and r, e, dr are the average radius of the transition sectionaround the center C′, the thickness and the width of each infinitesimalstep i, respectively. The power Po, by unit of surface is then given by:Po=(Ua−Ub)²×e/(ρπ²r²). The profile can be calculated with the objectivethat the power dissipated by each “step” is equal to the same value,whatever the step considered. When ρ has an even value throughout thematerial, which is generally the case, it does not interfere with theprofile calculation.

The thickness profile is such that the thickness of the transitionsection (4) is the largest (i.e. thickest) close to the peripheral edge(8) of the said section, and the smallest (i.e. thinnest) close to theinternal edge (7), i.e. close from the center C′ of the said section.Such a configuration allows to get a more homogeneous surfacetemperature with a negligible impact on the mechanical strength of thetransition section. In a simplified version the profile is such that thethickness E decreases steadily between the peripheral edge (8) and theinternal edge (7) of the transition section (4).

It has been found advantageous enough to use a variation of theinvention according to which the thickness varies discontinuously, with“steps”, between the peripheral edge (8) and the internal edge (7) ofthe transition section (4). In other terms, the thickness profilecomprises a limited number of sections, each of them being characterizedby a uniform thickness (E1, E2, . . . ) but different for each sectionor each “step” (M1, M2, . . . ). The width (L1, L2, . . . ) of each stepcould vary. The thickest part is normally located at the peripheral edge(8), which allows to keep high mechanical characteristic. This variationgreatly simplifies the fabrication of the resistor according to theinvention.

FIG. 5 illustrates a resistor according to this preferred variation ofthe invention. The A–A′ cross section of the FIG. 5A (seen in FIG. 5B)shows a step by step decrease of the thickness from the peripheral edge(8) to the internal edge (7) of the transition section (4). According tothis embodiment, the thickness profile has a similar shape for all thelines To, Tb, . . . included between the lines <<a>> and <<b>> and thethickness is approximately steady in each straight section (3). Theprofile could be symmetrical relative to the plane which contains thetransition section. The fabrication is easier with an asymmetric profileas illustrated in FIG. 5B, where one of the faces of the transitionsection is flush with one of the faces of straight legs (3).

The thickness profile according to this preferred variation can becalculated using the mathematical formulas proposed above. The steps “i”have then a finite width (L1, L2 . . . ) which is typically between 1and 20 mm. The number steps is limited, typically between 3 and 10steps. Too low a number does not allow to get the desired homogeneity,too high a number leads to excessive production costs of the crenellatedresistor.

The thickness in the straight sections (or “legs”) is typically between5 and 40 mm. In this preferred variation of the invention, in transitionsections, the thicker part of the profile is typically between 25 and 40mm. The thinner part, located next to the internal edge (7) has athickness which is 2 to 10 times smaller (typically approximately 5times lower) than the thickness of the thickest area located next to theperipheral edge (8) of the transition section.

It has also been found that, in order to achieve a good temperaturehomogeneity, it is not mandatory to have thickness variations anywhereelse than in the transition sections (improvement brought with a varyingthickness in the straight sections (3) are negligible). In theembodiment illustrated in the FIG. 5 the height of the step between thetransition section and the straight leg increases stage by stage fromthe peripheral edge to the internal edge. The inventor noticed that asmoother variation of the thickness between the transition section andthe straight leg had little influence on the distribution of theelectrical flow lines and that the resulting slightly improvedmechanical strength was not worth it when compared with the additionalcost resulting from the machining necessary to obtain such smoothsurfaces.

The peripheral edge (8) and internal edge (7) can be in whole or in partstraight or arcuated. They can also include some flat or curvedportions.

The inventor has determined, through a finite element calculation givingthe distribution of the lines of electric flow, the amount of powerdissipated per unit area (i.e. the power received and emitted by eachsurface unit) in the transition sections of a crenellated resistoraccording to prior art with a uniform wall thickness, and found that theratio between the largest and the lowest values of the power dissipationdensities is more than 200 over the surface of a transition section(typically between 50 μW/mm² à 15 mW/mm²). It has also found, with thesame calculation principles, that in similar conditions the ratiobetween the largest and the lowest values of the power dissipationdensities is no more than 10, or even no more than 5, when thetransition sections are made according to the invention with a thicknessprofile comprising only 4 steps with 4 different thicknesses. A profilewith 10 steps allows to keep this ratio between the highest and lowestlocal power below 3.

The carbonaceous material can be selected among the group comprisinggraphite materials, carbon materials and carbon/carbon compositematerials.

The crenellated resistor according to the invention can be produced byany known technique used to manufacture parts or items from carbonaceousmaterials. Typically, the production process for manufacturing aone-piece crenellated resistor includes:

-   the production of a hollow cylinder whose wall thickness has a    uniform value,-   machining operations (including if necessary drilling, cutting,    carving) of the wall of the cylinder so as to get the desired    meanders and the desired thickness profiles.

The hollow cylinder can be produced from a plain cylinder through somedrilling, coring, or cutting operations, or any other means.

The resistor can be made starting with a green block or cylinder andperforming at least one heat treatment step within the process.

1. A crenellated resistor made from a carbonaceous material and shapedas a hollow cylinder having a wall including a plurality of straightslots, said slots being alternated and regularly spaced to form ameander-like contour, said wall being thereby formed into a plurality ofelementary segments, each of said elementary segments including twosubstantially straight legs and a transition section therebetween havinga peripheral edge and an internal edge, wherein each said transitionsection has a thickness which decreases monotonically from theperipheral edge to the internal edge, and each of said straight legs hasa thickness which is substantially constant, whereby electric flowthrough the resistor results in a substantially even surfacetemperature.
 2. The resistor of claim 1, wherein the thickness in thetransition section decreases in steps of uniform thickness.
 3. Theresistor of claim 2, wherein the number of said steps is between 3 and10.
 4. The resistor of claim 2, wherein each transition section has athickness adjacent the internal edge which is 2–10 times smaller thanthe thickness adjacent the peripheral edge.
 5. The resistor of claim 1,wherein said carbonaceous material is selected from the group consistingof graphite materials, carbon materials and carbon/carbon compositematerials.
 6. The resistor of claim 1, which is a one piece crenellatedresistor.
 7. The resistor of claim 1, wherein the thickness in thetransition section has an asymmetrical profile, with one face in thetransition section being flush with a face of the straight legs.
 8. Afurnace suitable for the growth of silicon monocrystals and including atleast one crenellated resistor according to claim 2.